1. Field of the Invention
The present invention relates to a method of manufacturing a tuning fork type quartz resonator, quartz resonators manufactured by the above method of manufacturing, and electronic parts including the quartz resonators.
2. Description of the Related Art
The tuning fork type quartz resonators have long been adopted as a signal source for pacing a wrist watch owing to its being compact, inexpensive and having a low power consumption, and the uses thereof are still expanding.
FIG. 15 shows an example of the tuning fork type quartz resonators. In the drawing, the quartz resonator 1 is provided with a quartz piece (quartz blank) 10 including a base 11, two pieces (a pair) of vibrating arms 12 (12a and 12b) extending in parallel with each other leaving a space from the upper end side of the base 11.
Both main faces of the vibrating arms 12a and 12b are provided with grooves 13 and 14 respectively, which serve to enhance oscillation efficiency and suppress power loss. Excitation electrodes for exciting tuning fork oscillation based on bending vibration are formed in these grooves 13, 14, and on both main faces and both side faces (surfaces toward right and left of the drawing) of the respective vibrating arms 12a and 12b. 15 in the drawing is a crotch portion put between the respective vibrating arms 12a and 12b of the base 11, and the crotch portion 15 is a portion where no electrode is formed. The excitation electrodes are omitted in FIG. 15.
An outline of a conventional manufacturing process of the tuning fork type quartz resonator 1 will be explained here. First, masks are formed, for instance, on both front and back surfaces of, for instance, a Z-cut wafer made of quartz. Then, a large number of mask patterns along the outer shape of the quartz piece 10 and overlapping each other are formed on the masks on both front and back surfaces, and the quartz of the wafer surface is exposed along these mask patterns. Thereafter, wet etching is conducted so that the quartz exposed along the mask pattern is etched from front and back to form an outer shape of the large number of quartz pieces 10 simultaneously. A part of the periphery of the quartz piece 10 is remained as a supporting part without being etched, and the quartz piece 10 is held to the wafer by this supporting part at this time.
After the above-described etching, a metal film is formed by the sputtering method or the like so as to cover the respective quartz pieces 10, then a photosensitive photoresist is applied on the metal film to form a resist film. The resist film is exposed and developed to form a resist pattern so that the metal film is exposed along the pattern of the excitation electrode of the quartz resonator 1. Since the excitation electrodes are formed on the side walls of the vibrating arms 12a and 12b of the quartz piece 10 as explained in the structure of the quartz resonator 1 at the time of forming the resist pattern, it is necessary that the resist remain on the side wall of the quartz piece 10 so as to mask the metal film on the side wall. Therefore, since the side wall of the quartz piece 10 is hard to be exposed compared with the surface thereof, a positive type resist is used as the above-described resist, of which exposed part is changed into soluble and removed at the time of development.
After forming the resist pattern, the metal film is etched along the resist pattern to form excitation electrodes, so that the quartz resonator is manufactured. Thereafter, the resist film is removed and the quartz resonator is cut off from the wafer.
Incidentally, a wafer made of quartz has anisotropy, and when conducting etching, the etching is progressed at a different speed depending on the axis direction of the crystal axis as described in Patent Document 1. Therefore, when the outer shape of the quartz piece is formed by conducting etching from both front and back as described above on the Z-cut wafer, the etching does not progress along the thickness direction of the quartz wafer straightly but spreads in the lateral direction and, as a result, the crotch portion 15 is formed in front and back mirror symmetry. FIGS. 16A and 16B show an example of the front and back shapes of the crotch portion 15 thus formed. Protrusions are formed in a cone shape at the front and back surfaces of the crotch portion 15, and since the protrusions on the front surface side and on the back surface side are formed in mirror symmetry, it is formed in a complicated shape as if a plurality of protrusions are strung in a row seen as a whole of the crotch portion 15.
Furthermore, the tuning fork type quartz resonator 1 has made considerable progress toward downsizing, and following to this tendency, improvement in coverage property of the resist over the side wall at the time of forming electrodes has been requested. So, although the conventional resist application is mainly conducted by what is called spin coating in which while resist is being supplied to the center of a wafer, the wafer is rotated around its vertical axis so that the resist is spread owing to the centrifugal force produced by the rotation, an electrostatic spray method in which resist is jetted from, for instance, a spray nozzle and at the same time an electric charge is given to the resist and wafers so as to enhance the adsorbability of the resist to the quartz piece 10 in the wafer, a dip method in which the quartz piece 10 is immersed into resist liquid, and other methods have been used instead of the spin coating method. The electrostatic spray method will be explained in detail when the present invention is explained.
The crotch portion 15 of the quartz resonator 1, however, is formed complicatedly as described above, and liquid is sometimes apt to stay in a recess formed between protrusions of the crotch portion 15. Accordingly, when the above-described electrostatic spray method or the like is used, anxiety that the following problems might take place comes into existence. FIG. 17A shows the crotch portion 15 when resist is supplied, and 16 in the drawing indicates the resist film, and 17 in the drawing indicates the metal film to compose an excitation electrode.
After supplying the resist, the resist film 16 thus formed is exposed according to the resist pattern as described above. The crotch portion 15, on which electrodes are not to be formed originally, is also exposed, but the resist film 16 is sometimes formed thick on the crotch portion 15 due to the accumulated resist as described above, which may result in occurrence of fear of not being exposed to the extent of the lower layer of the resist film 16. In addition, there is a fear that unexposed portions are created because exposure beams are blocked at the crotch portion 15 due to its complicated shape. In such a case, the resist film 16 sometimes remains at the crotch portion 15 after development as shown in FIG. 17B. When the resist film 16 remains as described above, the metal film 17 below the resist film 16 is not etched. As a result, the metal film 17 remains on the crotch portion 15 after the excitation electrode is formed as shown in FIG. 17C. Once, the metal film 17 remains on the crotch portion 15 which is a region where no metal film is formed in itself in this way, a short circuit occurs between the excitation electrodes of the respective arms 12a and 12b due to the metal film 17, which may result in lowering in yield of the quartz resonator 1.
The above-described Patent Documents 1 and 2 describe a method of managing the structure of the crotch portion 15 to get a cone shape by bonding two pieces of quartz sheets in a manner that respective polarities of the X axes, their crystal axes, are reversed, and forming a tuning fork type resonator from this clad metal in order to obtain an angular velocity sensor. However, there is no description of problems created due to change in the resist application method at the time of forming electrodes in this manner. Although the Patent Document 3 describes that the shape of the crotch portion of a tuning fork gets complicated by etching quartz, there is no description of a means to solve the problem described above.
[Patent Document 1] Japanese Patent Application Laid-open No. 2002-188922 (paragraph 0008 to paragraph 0010)
[Patent Document 2] Japanese Patent Application Laid-open No. 2005-98841 (paragraph 0033 and FIG. 1 to FIG. 3)
[Patent Document 3] Japanese Patent Application Laid-open No. 2004-15562 (paragraph 0006)